CA2044760C

CA2044760C - Low nox burner

Info

Publication number: CA2044760C
Application number: CA002044760A
Authority: CA
Inventors: John J. Van Eerden; Chad F. Gottschlich; Wayne C. Gensler
Original assignee: Selas Corp of America
Current assignee: Selas Corp of America
Priority date: 1990-10-04
Filing date: 1991-06-17
Publication date: 1997-01-14
Anticipated expiration: 2011-06-17
Also published as: EP0479414A1; CA2044760A1; US5044931A; JP2553267B2; JPH04227404A

Abstract

A gaseous fuel burner provides substantial reduction of nitrogen oxide content in the combustion gases by creating a screen of premix combustion products, by introducing secondary gaseous fuel for admixture with the screen, and by exposing secondary air to the mixture for reaction with the secondary gaseous fuel.

Description

~ 204476~ ~
~ NOX BURNI~R

Rar ---- OF THE lh~n Field of the Invention This invention relates to a burner, particularly to one for burning a qaseous fuel, and further relates to a method of burning a gaseous fuel in a manner to produce combustion gases havinq a low content of nitrogen oxide. Hereinafter, nitrogen oxides, which are primarily nitric oxide and nitrogen dioxide, are collectively referred to as "NOx".
Na~or environmental and other problems have been encountered in the production of flue gases containing high contents of NOx.
The NOx tends to react under atmospheric conditions to form environmentally unacceptable conditions, including the widely known ~h~ A known as urban smog and acid rain. In the United States and elsewhere, enviL- ~1 legislations and restrictions have been enacted, and more are expected to be enacted in the future, severely limiting the content of NOx in f lue gases .
In U.S. Patent No. 4,874,310, granted October 17, 1989 to Selas Corporation of America, the assignee hereof, a controlled primary air inspiration gas burner was disclosed, in which the introduction of control primary air was controlled in order to provide a substantial reduction of the content of nitrogen o es 2044;;~60 ~n. the flue gas. Such a burner includes extra plplng for the introductLon and control of the primary air, and this sometimes introduces expense and possible complications, especially in furnace installations ut ~ n~ a very large number of burners .
Other endeavors have been made to reduce the content of NOx in furnace flue gases but many have been found unattractive in view of their requirement of too much operator att~nt ~ on, and in view of the need for extremely attentive control in order to assure th~t there will be no vlolatlon of existing enviL~- Al laws. It is very important to be able to obtain a very substantial reduction of NOx content 80 that even in the event of operator error the environmental law will not be violated and the further op~r~tion of the plant and its equipment will not be en~oined by 51ov~ tal action .
It has been the general indication in the prior art for premix burners that reduced NOx contents can be obtained by avoiding secondary air, by using substantially entirely primary air, and by firing the burner as close as posgible to its maximum firing capacity. We have now discovered a surprising exception.

Ob~ects of the Inventions It is accordingly an ob~ect of the invention to provide a burner and a method of burning gas wherein eYree~ i n~ly low NOx contents are obtA ~ nAhl - in the exhaust gases . It is a further ob~ect of this invention to provide guch a burner and method wherein careful, .1~ cate and precige operator control is nn~nqc~38;~ry to achieve the desired low NOx flue gas content. Still another ob~ect of the invention is to provlde such a burner and method wherein the requirements for utilizing substAnt~ ly 100%
primary air, and ut~ n~ a firing rate substantially equal to the maximum available firing rate for the burner, are no longer T~t~Ce~38ary .
I~nother ob~ect of this invention is to provide a burner which not only provides r~-1ic~1 ly reduced NOx values for the flue gas but which provides very subst~nt ~ :~1 1 y increased burner output capacity .
Other ob~ects and advantages of this invention, including the simplicity, economy and easy operability of the same, and the ease with which burners may be introduced into new furnaces or retro-fitted into existing furnaces, will become apparent hereinafter, and in the drawings of which:
Drawin~Js Figure 1 i8 a sectional view showing a burner embodying features of this invention;
Pigure 2 is a partial end view, slightly enlarged, of the burner of Figure l;
Figure 3 is a partial sectional view similar to Figure 1, but showing a - '; f i-~d form of burner in accordance with this invention;
Figure 4 is another partial sectional view similar to Figures 1 and 3, showing still another 'ifled form of burner in accordance with this invention;
Figure 5 is an end view of a burner tip embodying features of this invention;

204~760 Figure 6 i8 a sectional view taken as indicated by the lines nnd arrows VI-VI whlch appear in Figure 5 through the burner tip of Figure 5; ~md Figure 7 is a sectional view taken through the furnace wall, showing schematically a ~ ' ~ f ~d installation of a burner in accordance with this invention.
12et~ ed 1:48O~r~t~ of the Invention It will be appreciated that the following descrlption is ~nt~n~ to refer to the gpecific forms of the inYention selected for illustration in the drawings, and i8 not intended to define or limit the invention, other than in the ~rp~n~ d claim~.
In ut~ in~ the terms "primary airn and ~ secondary air~ in this specification it will be understood that the expression primary air ' is intended to be directed to air premixed with the gaseous fuel in the burner, whereas the expression "secondary air ' iB intended to be applied to air mixed beyond the burner nozzle and not conducted through the body of the burner.
Turning now to the specific form of the invention illustrated in the drawings, and referring particularly to Figure 1, the number 10 indicates a furnace wall having an optional burner block 11 forming an opening for insertion of the burner 12. The number 13 designates a secondary air shutter. Tube 14 of the burner 12 i8 threaded to a burner body expan~ion portion 15 which in turn is threaded into a burner tip 16. The furnace casing 17, support arms 20 and mounting plate 21 are provided in order s<L u~;Lu- ~lly to support the burner body tube 14.

Nounted on the Lnlet end of the burner body tube 14 is a threaded connection 22 for the lntroduction of n gaseous fuel into an inlet tube 23 . r-- i n~ gaseous fuel 18 conducted through a spud 24 provided with a multiplLcity of orifices 25. The orifices form ~ets of fuel which entrain primary air through the opening 26 by the well known ~et effect.
The fuel flowing throuqh the orifices 25, called the primary fuel, mixes with the primary air in order to form a gaseous premix, which iB caused to flow in the direction of the arrow (a) within the burner body tube 14, the expansion tube 15 and the premix chamber 28. In the usual manner, the premix is caused to flow through the burner tip 16 and out through the premix ports 27 for combustion within the furnace space at a location close to the inner surface of furnace wall 10 and the burner block 11.
In accordance with this invention, means are also provided for introducing secondary gaseous fuel into the burner tip. A gaseous fuel tube 30 is arranged to receive gaseous fuel within the inlet tube 23. Supported by the spud 24, it extends forwardly to the burner tip, and the secon~lAry gaseous fuel is conducted axially as shown in Figure 1 of the drawings. At its forward end the secondary gaseous fuel tube 30 is threaded at 32 into the burner tip 16, and cl ir~ates with a chamber 33 centrally formed in the burner tip 16 . The chamber 33 is in f luid communication with a plurality of PA~s~AS~ y~ 34,34 which are bored through vanes formed at the tip portion of the burner tip 16, which vanes will be described in further detail hereinafter. In the manner discussed, 2044 76(~
~conA~ry gaseous fuel i8 introduced through the gaseous fuel tube 30 into the chamber 33 and outwardly through a multiplicity of r~sag~_y~ 34 into the furnace space immediately ad~oining the inner wall of the furnace 10 and the burner block ll.
It will further be appreciated that, in view of the provision of a space 35 between the outer surface of the burner tip 16 and the inner surface of the burner block 11, an annular p~s~ y 35 is provided for the flow of secon~ry air, which seconrl~ry air is provided for the combustion of the secondary gaseous fuel emanating from th~ p~s~ _yD 34 into the furnace. Turning now to Figure 2, which L~p e~e..Ls a fr~ ry end view of the burner tip, it will be seen that the secondary gaseous fuel passageways 34 are drilled through a plurality of vanes 36 emanating from the central portion of the burner tip and ~Y~n.lin~ outwardly to the periphery of the burner tip 16. The vanes 36 are shaped to provide a plurality of spaced-apart premix ports 27 in the burner tip 16 for the premix. secause of the fact that the vanes 36 diverge from each other they define a divergent path for the flows of premix coming from the burner tip, and cause admixture with the secondary gaseouD fuel coming from the sec~)nA~ry gaseous fuel passageways 34.
As is shown in Figure 1, the flow (a) of premix is accordingly mixed with the flow (b) of the s~conA~ry gaseous fuel and the two are eventually mixed with each other and with the flow (c) of secondary air entering through the passageway 35. In the form of the invention shown in Figure l, the flows (a) and (b) of premix and secondary gaseous fuel enter the furnace at approximately the s~me distance away from the inner face of the furnace 10 and the hot face wall of ths burner block 11, snd the flow of secondary ~Lr (c) from the s~con~Ary alr pARsas~way 35 contacts both the combustion products from the premix and the secon~lAry fuel from the p~RsA~ways 34. It has been di~cuve:Led that the presence of the combustion products from the premix tempers the reaction between the secon~ry gaseous fuel and the secondary air flowing from the pARsa~; _y 35, and this is an important and advAn~g~o~R feature of the invention in si~JnificAntly reducing the NOx content of the resulting combustlon PL~IdU~; ~8 .
Continuing with reference to Figure 1, the diameters of the passageways 34 and the gaseous fuel tube 30 can be selected in concert with the number and dlameter of the orifices 25, to fix the ratio of primary gaseous fuel to secondary gaseous fuel. Since the primary gaseous fuel and the secon~Ary gaseous fuel are provided from the same source, namely inlet tube 23, a change of the diameters of passageways 34 and gaseous fuel tube 30 in concert with the number and diameter of orifices 25 results in a predet~rmin~d change of the ratio of primary gaseous fuel to the secondary gaseous fuel. This ls an advantageous feature of the 20447~0 invention since it reduces or eliminates the need for precise and individual control on the part of the operator.
Turning now to Figure 3 of the drawings, parts similar to those In Figure 1 are similarly ' - I:d and need not be ~ 8~ r~h~ d again, but in Figure 3 the passageways 34 are located forwardly along the burner tip, farther away from the burner block 11, than ~re the premix ports 27 of Figure 1. In this manner the premix combustion p-odu~;Ls coming from premLx ports 27 of Figure 3 form a combustion products screen 37 interposed between the path (c) of the secon~lAry air and the path (b) of the s~con~lAry gaseous fuel.
In this configuration the flow of combustion products along the path 37 interferes, at least to some extent, with any immediate admixture of the ~econdary gaseous fuel flowing in the path (b) and the secondary air f lowing in the path ( c ) . This tempers the reaction between the secondary gaseous fuel and the sec~n~lAry air, thus creating, in a manner not completely or fully understood, to a highly significant and entirely reliable reduction in the nitrogen oxide content of the flue gas resulting from the combustion. Similar effects are obtained, as will be apparent, in the ' ~ shown in Figures 1 and 2, and the relative spacing inwardly and outwardly of the respective passageways can be varied in a manner to produce particular tc-rnrr~rin~ effects for particular installations and desired combustion product compositions.
Turning now to Figure 4 of the drawings, it will be seen that the premix ports 27 are located inwardly, farther into the furnace, than are the secondary gaseous fuel passageway~ 34,34. While this 2()'14~50 provides a somewhat cloger relationship between the gec~n~l~ry qaseous fuel and the s~con~7~ry air flowing from the pIn8l~c~ y 35, and may in some cases not be preferred, the t~rh~ nt effect of the combustion products screen 37 nevertheless tempers the reaction between the secor ~l~ry gaseous fuel and the sernn~ ry air .
Turning now to Figure 5 of the drawings, it will be apparent that the burner tip includes a plurality of vanes 36, each carrying a s~con~l~ry gaseous fuel p~ s~ y 34, and that the8e p~rirh~ri~ll ly extend at spaced-apart locations around the periphery of the burner tip. Similarly, Figure 5 shows the angular relati~n~hir of the sidewalls 40 of the vanes 36.
Figure 6 shows one particular form of the burner tip, showing the manner in which the premix flow is achieved from the premix chamber 28 to form the combustion products screen 37.
Figure 7 shows a burner "L~u~ Lul~ similar to that of Figure 1, wherein the hot face of the burner block 11 extends farther into the furnace than does the inner face of the furnace wall 10. This is an important and advantageous feature of the lnvention. This feature enables the burner tip to be moved more deeply into the furnace, and this surprisingly provides an unexpected reduction of the NOx content of the resulting combustion products. Further, if the burner block 11 and the furnace wall 10 are flush with each other it is difficult to position the burner tip more deeply into the furnace without sacrificinq the ability to light the burner in the first instance. It has been surprisingly found that with the invention as shown in Figure 7 of the drawings it is not only ~ossible to provide a configuration resulting in a gi~nificAnt reduction of nitrogen oxide content in the flue gas, but also that this is readily a~ hP-i without any dif f iculty in initially 1 ~ht~ng the burner. We have obseLv~d that it is advantageous to maintain the burner relatively close to the burner block, 8uch as one inch or less, while moving the burner block farther into the furnace wall, and it is this combination which is believed to lower the NOx content of the flue gas without interfering with the ease of lighting of the burner.
Secondary air flow is controlled by the cross-sectional area of the passageway 35, furnace draft, and the position of the secondary air shutter 13.
Although it has been understood from past PYr~riF~nr e that the use of primary air alone ~L~nluces lower NOx, it is a surprising advantage of the burner in accordance with this invention that it ~;eemingly inconsistently provides a combination of primary and s~con-lAry fuel. This not only surprisingly reduces the NOx content in the combustion gases, but also radically increases the burner capacity even for the same size burner. This is attributable to .the fact that secondary gaseou8 fuel is being burned in addition to the primary gaseous fuel.
It is important in accordance with this invention that the premix introduced through the combustion passageways 27 forms a screen of burned gases which dilute the admixture of seron~lAry gas and secondary air, slowing the secondary fuel reaction rate. It is believed that this act of slowing the reaction rate results in a 2044 76~
de~reased flame temperature, whlch in turn results in a lower NOx content in the combustion gases. A substantial amount of a diluted mixture of burned gases and secondary fuel meets the secondary air, and it is believed that this rh~ occurs before the gecon~ ry air can directly and guickly react with the secondary gaseous fuel.
It has been discuvt~Le:d that this invention has another advantage in being surprisingly insensitive to the ~Le sence of excess air, 80 far as the NOx content of the emitted exhaust gas is con~ ~rn~d. In other words, the invention is highly valuable in connection with a furnace having air leaks allowlng for the presence of tramp air within the furnace chamber. It is believed that the use of a mixture partially composed of nozzle mixed, primary gaseous fuel and primary air, combined with a portion of secondary gaseous fuel and se~on~?~ry air, provides a more gentle slope to an NOx generation curve wherein NOx generation is plotted against the air-to-fuel ratio. This rh~ was u..~e~;Ced but has been found to provide a surprising lack of gensitivity to the p 3senc~ of undesired excess air, or tramp air, in the furnace chamber .
It is a further and important advantage of the invention, as will now be apparent, that in the operation of burners and in the combustion of gas in accordance with this Invention, all of the desired f low rates and ratios can be achieved and constantly held without the continuous control of operating personnel. In this way the achievement of a substantially constant and extremely low NOx ratio in the combustion gas can be achieved 1 n, i~r~n~ ntly of any ~Cari2tions that might otherwise be introduced by pé~r operating the burners or furnace.
The method of operl~tion of the burner in accordance with thiD
invention will now become apparent. The burner is ignited in the usual manner with the u8e of the gas supply. By ut 11 i ~1 n~ the in8plration characteristic8 of the spud 24, primary ~ir is taken in through the primary air p~age~:_y 26, and the burner is operated in a manner to i-~t ~oduce combustible premix through the premix ports 27. C~n~.uLL~ l.ly~ the gaseous fuel also flows through the secondary gaseous fuel tube 30 and radially outwardly through the seco~ ry gaseous fuel p~sase-~_yD 34. Automatically, and without requiring operator intervention, the secondary gaseous fuel mixes at least partially with a screen of premix or of premix combustion products, and the resulting mixture mixeD with the f low of secondary air in a moderate and controlled manner, resulting in combustion of secondary gaseous fuel with secondary air in the environment of the screen of premixed combustion produCts.
Although this invention has been described with reference to various specific c ' ~ I 8, it will be appreciated that many variations may be made without departing from the spirit and the scope of the invention. For example, various ~et nozzle cc~nfigurations may be resorted to, u~ili7in~ various numbers of passageways for the gas supply and for the supply of air, and with either gas or air pARsA~g~s ~,u~ ~lu~ding the other. For example, various a-~ r, - 8 may be made with regard to the sizes and ~hapes of the passa~ _y~ 27, 34, and even the secondary air passag~ 35. While the secnn~l~ry gaseous fuel tube 30 has been shown as centrally and axially ~rr~n~ed, lt will be apparent that a wide variety of other .9rr~1n~, ~ ts may be resorted to. Further, inste~ld of generating a primary fuel, primary air premix at the burner, premix already formed elsewhere may simply be directed into the premix chamber 28 of the burner tip.
The invention also applies to unidirectional burners pro~ecting combustion products in a cnnfin~d path, as well as 360 burners of the type illustrated in Fig. 5 of the drawings.
The use in the claims of the word wall" is not intended to be limited to a vertical wall but applies as well to floor or roof surfaces, or to slanting or walls otherwise arranged.
A5 will be apparent, any number and variety of shapes of primary air ports may be provided, as well as inlets for secondary air, and burners may be provided with or without the utllization of a secondary air shutter or secondary air passage, 80 long as a passageway such as passageway 35 is provided for delivery of secondary air to the burner tip. Many other variations may be made, as wLll be apparent to those skilled in the art, all without departing from the spirit and scope of this invention as defined ln the appended claLms. 13

Claims

1. A gaseous fuel burner comprising a burner body having a burner tip adapted to be installed in a furnace or the like for combustion of gaseous fuel therein, a primary supply means for introducing a primary gaseous fuel and primary air forwardly into said burner body for combustion with each other at a predetermined location along said burner tip;
secondary air supply means forming a secondary air passage extending along said burner body and having a secondary air outlet opening which is at a location spaced from said predetermined location on burner tip, and secondary gaseous fuel supply means forming a secondary gaseous fuel passageway extending along said burner body and having an outlet opening at said burner tip, whereby at least some mixing occurs at the burner tip between the primary air and gaseous fuel combustion products and the incoming secondary air before said secondary air undergoes combustion with said secondary gaseous fuel, whereby said burner produces combustion gases of reduced nitrogen oxide content.

2. The burner defined in claim 1 wherein said burner tip includes a primary outlet opening for releasing said primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said secondary fuel supply outlet opening is closer to the end of the burner tip than is said primary outlet opening.

3. The burner defined in claim 1 wherein said burner tip includes a primary outlet opening for releasing said primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said secondary fuel supply outlet opening is at about the same distance from the end of said burner tip as is said primary outlet opening.

4. The burner defined in claim 1 wherein said burner tip includes a primary outlet opening for releasing said primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said secondary fuel supply outlet opening is farther from the end of the burner tip than is said outlet opening.

5. The burner defined in claim 1 wherein both said primary and secondary gaseous fuel supply means are connected to a common source

6. The burner defined in claim 5 wherein control means are provided in said primary and secondary fuel supply means to control the relative rates of supply of said primary and secondary gaseous fuels to said burner.

7. The burner defined in claim 6 wherein said control means includes passageways of controlled sizes relative to each other.

8. The burner defined in claim 1 wherein said secondary fuel supply means includes a longitudinally arranged supply tube extending lengthwise of said burner and a plurality of connecting passages extending crosswise of said burner tip.

9. The burner defined in claim 1 wherein said primary air supply means includes an inspiration nozzle adapted to premix said primary gaseous fuel and said primary air.

10. In a furnace for burning a gaseous fuel, said furnace having a wall with inner and outer wall surfaces and a burner extending through said wall into such furnace for burning gaseous fuel with reduced production of nitrogen oxides in the resulting combustion flue gas, the combination which comprises:
(a) a burner block extending through said furnace wall surrounding said burner with intervening space between said air tube and said burner for flow of secondary air;
(b) said burner block having a hot face which extends into said furnace beyond said inner wall of said furnace;
(c) said burner having a tip which extends into said furnace beyond said hot face of said burner block;
(d) means at said burner tip for providing a combustion gas screen directed generally along said hot face of said burner block; and (e) means at said burner tip for introducing a separate flow of secondary gaseous fuel for flow generally along said hot face of said burner block, whereby said secondary gaseous fuel is mixed with both said combustion gas screen and the incoming secondary air from said intervening space.

11. The furnace defined in claim 10 wherein said burner block is composed of high-temperature material.

12. The furnace defined in claim 10 wherein said burner includes a supply of primary air and of primary gaseous fuel and further includes a supply of secondary gaseous fuel and secondary air.

13. The furnace defined in claim 10 wherein said gaseous fuel and air supplies are so positioned on said burner that at least some mixing occurs among the primary gaseous fuel, the primary air and the secondary gaseous fuel before said secondary gaseous fuel undergoes substantial combustion with said secondary air.

14. The burner defined in claim 10 wherein the burner is provided with a tip having an end spaced from the furnace wall, said tip having a primary outlet opening positioned for releasing primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said tip has a secondary fuel supply outlet opening positioned closer to the end of said burner tip than is said primary outlet opening.

15. The burner defined in claim 10 wherein the burner is provided with a tip having an end spaced from the furnace wall, said tip having a primary outlet opening positioned for releasing primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said tip has a secondary fuel supply outlet opening positioned at about the same distance from the end of said burner tip as is said primary outlet opening.

16. The burner defined in claim 10 wherein the burner is provided with a tip having an end spaced from the furnace wall, said tip having a primary outlet opening positioned for releasing primary gaseous fuel and primary air or mixtures or combustion products thereof into said furnace, and wherein said tip has a secondary fuel supply outlet opening positioned farther from the end of the burner tip than is said primary outlet opening.

17. In a method of heating a furnace with a gas burner located adjacent to a furnace wall, to provide a reduced percentage or nitrogen oxide components in the furnace flue gas, the steps which comprise :
(a) burning a mixture of primary gaseous fuel and primary air in said furnace;
(b) guiding the products of combustion of step (a) to form a combustion gas screen directed generally along a portion of said furnace wall adjacent to said burner;
(c) introducing a separate flow of secondary gaseous fuel in a path also directed toward the area along said portion of said furnace wall; and (d) introducing secondary air along said burner, whereby said secondary air is contacted by and at least partially mixed with said combustion gas stream as said secondary air moves into position to react with said secondary gaseous fuel.

18 18. The method defined in Claim 17, wherein the secondary gaseous fuel path is spaced farther from said furnace wall than is the primary gaseous fuel path.

19. The method defined in Claim 17, wherein said secondary air is introduced, relative to said furnace wall, at a location closer than said primary gaseous fuel path.